The present invention pertains to a method for passivating semiconductor device contacts and, in particular, for passivating nitride-defined Schottky diode contacts and shallow emitter contacts in silicon integrated circuit devices.
In recent years, silicon nitride masks have become a sought-after expedient in the fabrication of integrated circuits. Originally, the art taught that silicon nitride masking layers should be applied directly onto silicon substrates. However, the silicon nitride-silicon interface placed high stresses on the underlying silicon substrate. These stress often produce dislocations in the silicon substrate which resulted in undesirable leakage current, pipes and other adverse effects on the electrical characteristics of the interface. In order to minimize these stresses, the art next taught that a thin layer of silicon dioxide should be formed between the silicon substrate and the silicon nitride layer. This approach has been relatively successful where this composite layer is utilized for passivation.
Since metallic contact must be made with the semiconductor material underlying the protective dielectric layers, various openings or windows are cut in the layers, usually by an etching process. After an opening is formed through the outermost layer with one etchant, a second etchant is introduced through the opening to expose the underlying substrate. During the application of the second etchant, some lateral etching activity occurs that results in an overhanging ledge of the outermost film surrounding the entire opening or window. This over-hanging ledge can have undesirable effects on the resulting semiconductor articles.
For example, Schottky barrier diodes are often formed on silicon bodies having double-layer insulators, such as silicon dioxide coated with silicon nitride. Etching of the underlying silicon dioxide, through an opening in the silicon nitride, to define the diode area on the surface of the silicon body causes undercutting of the silicon nitride. Then, when a metal contact is deposited through the defined nitride area, a region of exposed silicon surface remains around the metallurgy and under the overhanging silicon nitride layer. This exposed silicon surface can lead to instability in the Schottky barrier diode during operation.
In another example, shallow emitter junctions are often formed in silicon bodies having double-layer insulators, such as silicon dioxide coated with silicon nitride. When the substrate is opened and the emitter is formed by ion implantation a problem arises with proper passivation of the emitter-base junction. This problem is caused because the ion implantation occurs in a direction normal to the substrate. The ions are masked by the nitride and do not reach the region of the substrate under the overhanging nitride layer. Therefore, since the passivating layer is undercut beneath the mask opening, the layer will not properly overlap the implanted emitter-base junction at the surface of the substrate. As a result, the emitter-base junction may not be properly passivated and the device yields are reduced.
A large amount of activity has occurred in the art aimed at solving these problems. For example, an article entitled "Process Modification For Improved Bipolar Circuit Performance" by C. G. Jambotkar, in IBM Technical Disclosure Bulletin, Vol. 24, No. 11A April 1982, pp. 5574-5577, recognizes the problem that arises in device fabrication when sequential silicon dioxide and silicon nitride layers are formed over a substrate and then opened up to expose the substrate, stating at p. 5577: "It is well-known that in a comparable standard process incorporating emitter implantation, the SiO.sub.2 undercuts, which especially occur when wet SiO.sub.2 etching is used, frequently create excessive emitter-base leakage because of the inadequate passivation of the emitter-base junction at the silicon surface--especially if the emitters are attempted to be made shallow." The article suggests a solution to this problem by means of a two-step process. First, the undercut region is filled by chemical vapor deposition (CVD) of about 500 angstroms of silicon dioxide and about 300-500 angstroms of silicon nitride. This is followed by the reatcive ion etch (RIE) removal of this composite layer everywhere except in the undercut, filling the sidewall regions.
In a further example, an article entitled "Reliable Passivation of Shallow Emitters As Well As Nitride-Defined Schottky Diodes" by I. Antipov and C. G. Jambotkar in IBM Technical Disclosure Bulletin, Vol. 25, No. 9 February 1983, pp. 4782-4784, discloses that in some standard bipolar technologies, when emitter and Schottky diode windows are formed in composite layers of silicon dioxide and silicon nitride, undercuts of the silicon nitride layer are created due to the undesired etching of silicon dioxide. This causes unreliability of the devices, leakages of Schottky diodes and/or emitter-base junctions. The article suggests a solution at p. 4784: "Without removing the screen oxide or, alternatively, after removal of the screen oxide and a thin re-oxidation, Si.sub.3 N.sub.4 layer 12 is deposited to refill the undercut regions, . . . Through (vertically directional) RIE, Si.sub.3 N.sub.4 layer 12 is removed excepting its portions filling the undercut regions."
In a further example, an article entitled "Method Of Producing Schottky Contacts", by M. Briska and A. Schmitt in IBM Technical Disclosure Bulletin, Vol. 22, No. 11 April, 1980, states on p. 4964: "This method concerns the production of Schottky contacts on a silicon semiconductor substrate, which are laterally limited by a silicon nitride layer. . . . The Schottky contacts are produced by applying to a suitably doped semiconductor substrate 1 a silicon dioxide layer 2 followed by a first silicon nitride layer 3. Contact hole 4 is etched first into layer 3 and then into layer 2. When layer 2 is etched, layer 3 is laterally underetched, so that layer 3 overlaps, . . . When the contacts are subsequently vapor deposited, hollow spaces, which cannot be contacted and often lead to increased leakage currents, are formed under the overlapping layer part." The article suggests the following solution at p. 4964: "To eliminate this problem, the first layer 3, which is removed after contact hole 4 has been opened in layer 2, is replaced by a second layer layer 5 . . . having the same thickness as layer 2 arranged underneath it. This second layer 5 is then etched down to layer 2 by reactive ion etching (RIE), maintaining a nitride ring 6 . . . on the periphery of the contact hole."
A similar method for making stable nitride-defined Schottky barrier diodes by eliminating the undercut cavity in the oxide layer beneath a nitride ring defining the Schottky contact is shown in U.S. Defensive publication T101,201. It discloses filling the cavity by CVD depositing oxide into the undercut oxide cavity beneath the ring. The CVD oxide is then reactively ion etched to remove it except along the vertical walls of the nitride ring and the oxide cavity.
These methods suffer a drawback due to stresses caused at the silicon substrate-dielectric layer interface. Generally, a silicon dioxide layer on a substrate induces a compressive stress on the substrate. On the other hand, a silicon nitride layer in contact with a substrate induces a tensile stress with respect to the same substrate. Furthermore, the tensile stress per unit surface area created by a conventionally deposited silicon nitride layer with respect to the substrate is on the order of approximately 10 times the compressive stress per unit area created by a silicon dioxide layer. Therefore, the methods in the art of refilling the undercut, and especially those which leave a substantial nitride-substrate interface, would cause stress and produce defects and leakage in the devices.